13.4 Estimating Needs in Critical Illness & Special Clinical States

Why Critical Illness Breaks the Equations

A healthy child's energy requirement is the sum of resting metabolism, activity, growth, and the thermic effect of food. Critical illness dismantles that arithmetic in both directions at once. Inflammation, catecholamines, fever, and wound healing raise expenditure; sedation, neuromuscular blockade, mechanical ventilation, thermoneutral environments, and the temporary suspension of growth lower it. The net result is frequently close to, or even below, resting energy expenditure during the acute phase - which is why the traditional habit of multiplying a predicted resting value by a generous stress factor is the single most common cause of overfeeding in the pediatric intensive care unit (PICU).

Overfeeding is not benign. It produces hyperglycemia, excess carbon dioxide production that delays ventilator weaning, hepatic steatosis, hypertriglyceridemia, and azotemia. Underfeeding is equally real - cumulative deficits in the first PICU week predict longer ventilation and worse outcomes - so the specialist is aiming at a narrow target with poor instruments.

What the ASPEN/SCCM Pediatric Guidelines Say

The joint ASPEN and Society of Critical Care Medicine guidance for critically ill children establishes a set of positions the CSP exam expects:

• Measure when you can. Indirect calorimetry is recommended for determining energy requirements, especially in children with suspected altered metabolism, failure to wean from the ventilator, unexplained growth failure on calculated feeds, and burns or major trauma.
• If you must estimate, estimate carefully. Use Schofield or FAO/WHO/UNU equations without added stress factors. Explicitly avoid the Harris-Benedict equation and simple RDA/DRI-based estimates in this population - both were derived in populations that do not resemble a sedated, ventilated child.
• Deliver at least two-thirds of the prescribed energy goal by the end of the first week.
• Protein: at least 1.5 g/kg/day. This minimum is substantially above the age-based RDA and reflects the obligatory protein catabolism of critical illness; higher intakes (commonly up to 2-3 g/kg/day) are used in burns, wounds, and continuous renal replacement therapy.
• Enteral nutrition is the preferred route, started early (within roughly 24-48 hours) in the hemodynamically stable child, with stepwise advancement and a written protocol.
• Do not rush parenteral nutrition. The PEPaNIC randomized trial found that withholding supplemental PN for one week in critically ill children who were not already malnourished shortened ICU stay and reduced infections compared with early PN. Practice therefore delays supplemental PN in the low-nutrition-risk child while prioritizing enteral advancement; children with pre-existing malnutrition or a nonfunctional gut are handled individually.
• Screen for nutrition risk on admission and reassess, since the sickest children are the ones whose deficits accumulate fastest.

A final nuance the exam likes: in the sedated, paralyzed, ventilated child, the activity component is near zero, so starting near measured or predicted resting expenditure and titrating to tolerance, glucose control, carbon dioxide production, and weight trend is safer than any formula-plus-multiplier approach.

Special Clinical States

Burns

Burns produce the most extreme and most sustained hypermetabolism in pediatrics - resting expenditure can run 1.5-2 times predicted and remains elevated for months after wound closure. Pediatric-specific formulas exist because adult burn equations overestimate in children:

• Galveston formulas are body-surface-area based, for example roughly 1,500 kcal/m² total body surface area + 1,500 kcal/m² burned surface area per day for children, with age-adjusted variants for infants and adolescents.
• Curreri junior formulas combine maintenance requirements with a per-percent-burn increment.
• Protein 1.5-3 g/kg/day supports wound healing and limits lean mass loss.
• Early enteral nutrition (within hours) is standard, and micronutrients matter: zinc, copper, selenium, and vitamin C are lost through exudate and are routinely supplemented.
• Indirect calorimetry, repeated as the wound closes, beats every formula here, because the multiplier falls as grafts take.

Continuous Renal Replacement Therapy and Dialysis

CRRT removes small solutes continuously, and that includes nutrients. Amino acid losses commonly run about 10-15% of intake, so protein prescriptions rise - often to 2-3 g/kg/day in children on CRRT - and protein should not be restricted to control azotemia when a clearance therapy is running. Water-soluble vitamins (thiamine, folate, vitamin C) and trace elements (selenium, copper, zinc) are also lost and need surveillance. Citrate anticoagulation contributes calories as citrate is metabolized, and dialysate or replacement fluids may contribute glucose - both count against the energy budget.

Extracorporeal Membrane Oxygenation

Children on ECMO are hypermetabolic and generally tolerate enteral nutrition; the circuit itself is not a contraindication. Volume constraints and the calories contributed by sedatives and infusions must be counted.

Propofol and Other Non-Nutritional Calories

Propofol is delivered in a 10% lipid emulsion supplying about 1.1 kcal/mL. In a sedated child on a prolonged infusion this can silently supply a large fraction of the fat prescription; subtract it from the lipid dose and monitor triglycerides. Dextrose-containing maintenance fluids, citrate, and peritoneal dialysate glucose are the other common hidden sources.

Obesity in the Critically Ill Child

Adipose tissue is metabolically less active than lean tissue, so using actual body weight overestimates energy needs while ideal body weight underestimates them. The practical compromise is an adjusted body weight - typically ideal body weight plus 25-50% of the excess - for the energy calculation, while protein is dosed to preserve lean mass rather than being scaled down with the energy target. Indirect calorimetry is especially valuable here because equation error is largest at the extremes of body size.

Fever, Trauma, and Traumatic Brain Injury

Fever raises resting expenditure by roughly 10-13% per degree Celsius above 37 °C. Traumatic brain injury raises expenditure acutely but is blunted by sedation, paralysis, and barbiturate coma - one of the clearest illustrations that the therapy, not just the diagnosis, sets the number. Across all of these states the same discipline applies: prescribe from the best available measurement, reassess frequently as therapy changes, and treat any formula output as a hypothesis to be tested against glucose, carbon dioxide, triglycerides, nitrogen balance, and the growth chart.